Typically precious metal containing ores are leached with cyanide as the most efficient leachant or lixiviant for the recovery of precious metal values from the ore.
However, because of the mineralogy of various ores, access of the precious metal in the ore, by cyanide is low, for an economical extraction of the precious metal values in an ore. If the cyanide extraction produces small or negligible amounts of gold, an ore is said to be refractory or highly refractory. Various methods have been employed to increase the extractability of the precious metals. A good summary article describing the prior problems is that authored by Kantopoulos et al. Process Options for Refractory Sulfide Gold Ores: Technical, Environmental, and Economic Aspects, Proceedings EPO '90 Congress, D. R. Gaskell, Editor, The Minerals, Metals & Materials Society, 1990.
A typical component which causes the refractoriness of the ore is predominantly a carbonaceous type component either inorganic or organic. The organic carbonaceous materials are also classified as acid insoluble carbonaceous materials. Gold found in ores dispersed within or occluded in a sulfide matrix may be considered refractory because of inaccessibility of such gold by cyanide leaching.
When treating such ores, the economic considerations dictate the selection of the process or the pretreatment of the ore to render it amenable first and foremost to cyanide extraction even though other gold lixiviants may be used.
As one of the desired treatment steps prior to cyanidation, roasting of ores in presence of air is typical. Lately oxygen or oxygen and air roasting, at low temperatures, have showed considerable promise. Other commercial ore treatment methods prior to cyanidation are high pressure oxygen and/or oxygen-ozone pretreatment, chlorine pretreatments, hypochlorite pretreatments and the like.
To improve cyanidation of ores, during such cyanidation ozone, or ozone and oxygen, or oxygen, or a surfactant, or combinations of these are also employed. Methods such as "carbon-in-pulp" or "CIP" and "carbon-in-leach" or "CIL" are used to improve cyanidation reactions and gold recovery.
However, cyanidation has certain shortcomings, primarily an ore material must be neutralized after an acid generating treatment as cyanidation must be carried out on the alkaline side of the pH scale; likewise high cyanide consumption renders a process less attractive. When using thiourea, neutralization of the ore is not as demanding and does not affect thiourea extraction of gold, but the extraction economies are impaired by the higher cost of thiourea and the reduced efficiency when compared with cyanide.
Other compounds which have been used and offer promise because of reagent costs are compounds such as thiosulfates of which ammonium thiosulfate is one of the desirable candidates. Although still other materials are used for gold recovery, these are not yet of industrial significance.
When ammonium thiosulfate and the like are used, neutralization of ore is required as appropriate pH ranges are neutral to alkaline, e.g. to about pH 7 to 10. As pyritic sulfidic ores need to be neutralized because of the acidity of these ores when subjected to oxygenation or biooxidation and like treatments, separate process steps are required.
Inasmuch as gold is occluded in the sulfide matrix of the ore, the accessibility by cyanide has sought to be improved for these ores; the same is also true when considering an appropriate sulfide, e.g. pyrite for oxidation or biooxidation. Although, various oxidation or biooxidation reactions have been tried such as vat, autoclave, slurry or liquid solution oxidations, these reactions are not practical when using large ore bodies having low gold content. As one of the approaches to oxidation of low content metal sulfide ores, biooxidation has come into prominence and much effort has been expended in research. Biooxidation was first applied to copper. Biooxidation of copper ore has been a well tried method although it is considered fairly slow.
When biooxidation is coupled with oxidative bioleaching, i.e. when direct, indirect and even galvanic leaching reactions are involved, some of the disadvantages of the slow biooxidation reactions are mitigated. Biooxidation reactions typically involve arsenopyrite and pyritic iron-sulfide containing ores including those that have some refractory carbon components present. Biooxidation, however, can suffer from inhibitory concentrations of some metals present in the ore. Biocidically active metals are such as arsenic, antimony, cadmium, lead, mercury, molybdenum. Ions such as chlorine, bromine and the like affect the biooxidation process. Because of slow growth rates for some bacteria as well as temperature variations in a typical ore dump undergoing sulfide oxidation, considerable efforts have been expanded to improve the rate constraints which have limited or held back the potentially very useful application of biooxidation.
Hence, considerable investigation has been made of the various limiting conditions concerning commercial biooxidation including such factors as ores in heaps or in slurry form, the use of surfactants, the use of potentiators or biooxidation promoters such as silver, aluminum, etc., appropriate selection and growing of robust bacteria which would be resistent to the inhibitory biocide activity of metals such as arsenic and growing the bacteria in perfuse amounts. Other considerations have been such as nutrient access, air access and carbon dioxide access for making the process even more efficient and thus an attractive ore treatment option. References illustrating these efforts are such as by Bartlett, Aeration Pretreatment of Low Grade Refractory Gold Ores, Minerals and Metallurgical Processing, pp 22-29, (Feb. 1990); Bennett et al, Limitations on Pyrite Oxidation Rates in Dumps Set By Air Transport Mechanisms, Biohydrometallurgy, Proceedings of Jackson Hole Symposium, Aug. 13-18, 1989 Canmet (1989); Burbank et al, Biooxidation of Refractory Gold Ore in Heaps, Ch. 16, pp 151-159 in Advances in Gold and Silver Processing, Reno Proceedings of Symposium "Goldtech 4", Reno, Nev., Sep. 10-12, 1990, Society of Mining, Metallurgy and Exploration, Publisher, 1990; Dix, Laboratory Heap Leach Testing: How Small and Large Scale Tests Compare, Mining Engineering, June 1989, Pages 440-442.
Amongst the methods seeking to improve biooxidation many methods have been proposed for mechanically increasing the access of the biooxidant bacteria to the ore. These methods have relied upon agitation of the ore either in tanks, slurries, providing circulation in vessels or reconstitution and remixing of the materials including stirring, raking, forming an improved slurry, transfer of slurry materials, providing stirred tank basins or have addressed various aspects of heap construction and utilization. References to such considerations are found in an article by Andrews, Large-Scale Bioprocessing of Solids, Biotechnology Progress, Vol. 6, pp 225-230, 1990.
Patents which illustrate some of these methods mentioned above are found such as in U.S. Pat. No. 4,324,764 concerning mechanical distribution of ores or distribution of ores by conveyors such as in U.S. Pat. No. 4,571,387 or a change in heap structure such as in U.S. Pat. No. 4,279,868 or stagewise heap formation such as in U.S. Pat. No. 4,017,309; or a stirred tank--semi "heap" construction such as disclosed in U.S. Pat. No. 4,968,008.
However, when treating large amounts of waste heap material or tailing material, the normal considerations that are applicable in high grade precious metal ore treatments are not viable. For waste ore treatment, economics often dictate a one-shot type of heap formation, e.g. for the depth, the size, the reactant accessibility, etc. Moreover, for biooxidation, the induction times concerning biooxidants, the growth cycles, the biocide activities, viability of bacteria and the like become important because the variables such as accessibility, particle size, settling, compaction and the like are economically irreversible once a heap has been constructed as such heaps cannot be repaired except on a very limited basis. For example, compaction problems such as are encountered in heap treatment of ores, and others such as puddling, channelling, or nutrient, carbon dioxide, or oxygen starving, uneven biooxidant bacterial distribution, and the like have been addressed in a number of investigations with respect to biooxidation. Such problems are also encountered in cyanide leaching.
For example, to solve channelling in percolation leaching by cyanides it is known to agglomerate the ore materials of high grade ores such as disclosed in U.S. Pat. No. 4,256,705 and 4,256,706. Other approaches to improve percolation leaching by cyanides include addition of fines such as flocculating materials, fibers, wood, pulp and the like as disclosed in U.S. Pat. No. 4,557,905. The last patent discloses leachable matrix formation to allow for access of cyanide to the precious metal values.
An ultimate, albeit impractical, suggestion for cyanide leaching has been found in U.S. Pat. No. 4,424,194 which shows making useful articles and then leaching these. This patent may have as its progenitor the early U.S. Pat. No. 588,476 of Aug. 17, 1887, which discloses porous casts made of gold "slimes" and gypsum. These casts are thereafter broken and leached.
Although for a variety of different reasons agglomeration has been practiced in the metallurgical arts such as in high temperature blast furnace art for various feed material preparations for blast furnaces, opposite suggestions have also been found concerning non-agglomeration and extraction of metals such as the pulp-liquid extraction described in U.S. Pat. No. 3,949,051. Extraction of the precious metals from heaps, preformation of heaps and heap treatment is found such as in U.S. Pat. No. 4,017,309 and 4,056,261.
Further improvements for access of cyanide to the precious metals have been described in U.S. Pat. No. 4,318,892 and 4,279,868 as well as U.S. Pat. No. 4,301,121. All of these attempts have sought to improve the distribution of the leachant or the mixing ratios of the ore to the lixiviant, but these attempts are typically addressed to providing better access for cyanide and to overcome the ostensible refractoriness of the ore. Other like disclosures have been found in U.S. Pat. No. 4,324,764 and 4,343,773.
Heap improvements have been found in the construction of the particles such as paste formation with the lixiviant and subsequent aging of the ore on treatment of the same, described in U.S. Pat. No. 4,374,097. Likewise, specific berm construction for the improved extraction of liquids from a specifically constructed heap has been found in U.S. Pat. No. 4,526,615. Similarly various particle specifications have been described for the ore particle treatment including the micro agglomerates of a size of 500 microns (and lower) found in U.S. Pat. No. 4,585,548.
In all of these heap formations, heap treatments or heap leaching methods, shortcomings have been sought to be overcome by the increase of cyanide efficiency such as by oxygen addition, e.g., in U.S. Pat. No. 4,721,526, or the use of various liquors in the recovery of gold described in U.S. Pat. No. 4,822,413.
Agglomerating agents for copper ores are shown in U.S. Pat. No. 4,875,935. Opening up clogged heaps has also been shown and discussed in U.S. Pat. No. 3,819,797 and heap treatment for distribution of a lixiviant is disclosed in U.S. Pat. No. 5,005,806. Finally, both conjoint crushing and agglomeration of ore has been discussed in U.S. Pat. No. 4,960,461.